A fuel cell device generates electricity directly from a fuel source, such as hydrogen gas, and an oxidant, such as oxygen or air. Since the process does not "burn" the fuel to produce heat, the thermodynamic limits on efficiency are much higher than normal power generation processes. In essence, the fuel cell consists of two catalytic electrodes separated by an ion-conducting membrane. The fuel gas (e.g., hydrogen) is ionized on one electrode, and the hydrogen ions diffuse across the membrane to recombine with the oxygen ions on the surface of the other electrode. If current is not allowed to run from one electrode to the other, a potential gradient is built up to stop the diffusion of the hydrogen ions. Allowing some current to flow from one electrode to the other through an external load produces power.
The membrane separating the electrodes must allow the diffusion of ions from one electrode to the other, but must keep the fuel and oxidant gases apart. It must also prevent the flow of electrons. Diffusion or leakage of the fuel or oxidant gases across the membrane can lead to explosions and other undesirable consequences. If electrons can travel through the membrane, the device is fully or partially shorted out, and the useful power produced is eliminated or reduced.
It is therefore an object of this invention to produce a membrane which allows the diffusion of ions, specifically protons, but prevents both the flow of electrons and the diffusion of molecular gases. The membrane must also be mechanically stable and free of porosity and pinholes which would allow passage of molecular gases.
In constructing a fuel cell, it is particularly advantageous that the catalytic electrodes be in intimate contact with the membrane material. This reduces the "contact resistance" that arises when the ions move from the catalytic electrode to the membrane and vice versa. Intimate contact can be facilitated by incorporating the membrane material into the catalytic electrodes. [See Wilson and Gottsfeld J. Appl. Electrochem. 22, 1-7 (1992)] It is therefore an object of the invention to produce a membrane wherein such intimate contact is easily and inexpensively made.
For reasons of chemical stability, fuel cells presently available typically use a fully fluorinated polymer such as Dupont Nafion.RTM. as the ion-conducting membrane. This polymer is expensive to produce, which raises the cost of fuel cells to a level that renders them commercially unattractive. It is therefore a further object of this invention to produce an inexpensive ion-conducting membrane.
Ion-conducting polymers are known. (See Vincent, C. A., Polymer Electrolyte Reviews I, 1987). Many of the known polymers are, for the most part, similar to sulfonated polystyrene because of the known ability of sulfonated polystyrene to conduct ions. Unfortunately, uncrosslinked, highly sulfonated polystyrenes are unstable in the aqueous environment of a fuel cell, and do not hold their dimensional shape.
U.S. Pat. Nos. 5,468,574 and 5,679,482 disclose an ion-conducting membrane composed of hydrogenated and sulfonated block copolymers of styrene and butadiene. Although fuel cells made from membranes of these sulfonated block copolymers have operating lifetimes that make them commercially viable, the mechanical strength of the block copolymer, when swollen with water at operating temperatures, ultimately limits the life of the fuel cell. An inexpensive, chemically stable, ion-conducting membrane with greater mechanical strength would therefore provide an improved operating lifetime for such cells.